Tansley Review No. 98 Tree and Forest Functioning in an Enriched CO$_2$ Atmosphere

• Published in: The New phytologist 1998-07, Vol.139 (3), p.395-436
• Main Authors: Saxe, Henrik, Ellsworth, David S., Heath, James
• Format: Article
• Language: English
• Subjects: Atmospherics; Carbon dioxide; Forest ecosystems; Forest soils; Forest trees; Photosynthesis; Pine trees; Plants; Seedlings; Trees
• ISSN: 0028-646X; 1469-8137

Forests exchange large amounts of CO$_2$ with the atmosphere and can influence and be influenced by atmospheric CO$_2$. There has been a recent proliferation of literature on the effects of atmospheric CO$_2$ on forest trees. More than 300 studies of trees on five different continents have been published in the last five years. These include an increasing number of field studies with a long-term focus and involving CO$_2$ x stress or environment interactions. The recent data on long-term effects of elevated atmospheric CO$_2$ on trees indicate a potential for a persistent enhancement of tree growth for several years, although the only relevant long-term datasets currently available are for juvenile trees. The current literature indicates a significantly larger average long-term biomass increment under elevated CO$_2$ for conifers (130%) than for deciduous trees (49%) in studies not involving stress components. However, stimulation of photosynthesis by elevated CO$_2$ in long-term studies was similar for conifers (62 %) and deciduous trees (53 %). Recent studies indicate that elevated CO$_2$ causes a more persistent stimulation of biomass increment and photosynthesis than previously expected. Results of seedling studies, however, might not be applicable to other stages of tree development because of complications of age-dependent and size-dependent shifts in physiology and carbon allocation, which are accelerated by elevated CO$_2$. In addition, there are many possible avenues to down-regulation, making the predicted canopy COg exchange and growth of mature trees and forests in a CO$_2$-rich atmosphere uncertain. Although, physiological down-regulation of photosynthetic rates has been documented in field situations, it is rarely large enough to offset entirely photosynthetic gains in elevated CO$_2$. A persistent growth stimulation of individual mature trees has been demonstrated although this effect is more uncertain in trees in natural stands. Resource interactions can both constrain tree responses to elevated CO$_2$ and be altered by them. Although drought can reduce gas-exchange rates and offset the benefits of elevated CO$_2$, even in well watered trees, stomatal conductance is remarkably less responsive to elevated CO$_2$ than in herbaceous species. Stomata of a number of tree species have been demonstrated to be unresponsive to elevated CO$_2$. We conclude that positive effects of CO$_2$ on leaf area can be at least as important in determining canopy